The present invention generally relates to a novel vibratory screening apparatus particularly adapted fo the wet-screening of slurry mixtures containing particles and liquids, and more particularly, to the separation of solid particles and liquids to effect substantial dewatering of the slurry mixture. The novel screening apparatus of the present invention utilizes in combination a combined vibratory screening deck having a sieve bend portion upstream of an inclined substantially planar portion to effect an improved substantial dewatering of a slurry mixture as the slurry flows across the screening deck.
The present invention is particularly useful in dewatering slurry mixtures of fine-grained ore particles, such as fine coal or the like, commonly encountered in coal and ore preparation facilities. Such fine-grained ore particles have intrinsic value, but must be substantially dried, and there is consequently a continued need for improved means to effect maximum separation of the particles and liquid in such slurry mixtures (e.g. to maximize the recovery of fine ore particles freed to a maximum extent of the separated liquid).
Typical coal preparation facilities have conventionally utilized a plurality of sieve bends for such dewatering and separating processes. The dewatered fine coal normally proceeds to further processing, such as driers for removal of additional moisture from the coal fines. However, conventional sieve bends may not remove sufficient water from the fine-grained coal to enable the driers to be operated in the most efficient manner and/or a substantial energy requirement is imposed for operation of such driers. That is, the driers must expend an additional amount of energy to reduce the moisture content of the fine coal feed thereto after it has been dewatered according to conventional devices.
While sieve bends are a well known apparatus utilized for separation and dewatering processes, conventional sieve bends may not separate the desired amount of liquid from coal fines nor attain the degree of separation so as to minimize the amount of coal fines lost to the underflow. The genesis of the sieve bend appears to be U.S. Pat. No. 2,916,142 issued to Freerk J. Fontein, the entire disclosure of which is incorporated herein by reference. According to conventional sieve bend design, the spacing or apertures between the bars of the sieve bend screen deck (arranged perpendicular to the direction of feed) determines the maximum particle size that will pass therethrough. The effective operation of conventional sieve bends is based on achieving a laminar flow of slurry mixture across the surface of the screen deck substantially undistributed since successive bars of the sieve bend engage succeeding layers of the slurry mixture removing water and particles from such layer to a depth of approximately 1/4 the aperture width, with particles passing therethrough having a diameter of less than 1/2 the aperture width. Disturbances of the laminar flow have generally been thought to effectively obviate the inherent separating correlation of a sieve bend between particle size and the bar spacing. Thus, sieve bends have not been contemplated in the art as really useful as a vibratory screening apparatus.
It has been proposed to utilize "knocker" or "rapper" devices having an extremely low frequency of timed intervals and at spaced points along the width of the sieve bends as evidenced by U.S. Pat. No. 3,446,349 to Benzon, primarily to dislodge large irregular particles which might get caught in and clog or blind the screen apertures. However, such knocker devices have experienced problems such as damage to the screen deck and the destruction of the correlation between aperture spacing and particle size that will pass therethrough. Rapper sieve bends, therefore, may undesirably allow particle sizes of greater than 1/2 the aperture width to pass through the screen cloth as underflow. As noted above, in coal fine dewatering processes, such an increase in the particle size that will pass through the screen cloth is unacceptable since the predictable result is that valuable fine coal particles are removed from the product overflow.
It has also been occasionally proposed to utilize very high frequency (e.g. hummer screens) oscillators in the sonic range in conjunction with a flexible filtering screen cloth for dewatering purposes as evidenced by U.S. Pat. No. 3,124,530 to Jakobs. However, the fine mesh flexible screen cloth of conventional hummer screens do not provide the advantageous qualities associated with the rigid bar cloth structure of sieve bends. Accordingly, the ruggedness demanded of equipment utilized in coal processing facilities is often not available when utilizing such conventional hummer screens.
According to the present invention, vibratory movement is utilized in combination with a special screening deck which comprises a combined sieve bend portion and an inclined generally planar screen portion. The present invention is concerned with utilizing vibratory movement between about 800 RPM and 1,200 RPM, preferably in the range between about 900 to 1,100 RPM. According to the present invention the feed end sieve bend portion accepts a slurry feed mixture of particles and liquid in a direction generally perpendicular to the axes of the bars which comprise the screen cloth of the screen deck.
The slurry mixture of particles and liquids is introduced at the feed end of the sieve bend portion and as the slurry flows between the feed end and discharge end thereof, it is effectively partially dewatered. Dewatering is a term applied to the process of removing liquid from a mixture of particles and liquid so that the percentage of liquid in the slurry at the beginning of the dewatering process is much greater than at the end of such process, e.g. from a dilute slurry to a concentrated slurry, and while a minimum of particles passes through to the underflow.
As the slurry becomes dewatered, a transition between particles entrained with liquid and liquid entrained with particles occurs. That is, the weight percent of particles present in the slurry becomes greater as the slurry proceeds across the sieve bend deck between the feed and discharge end. Thus, whereas a dilute slurry is introduced at the sieve bend feed end of the screening apparatus according to the present invention, a substantially but partially dewatered overflow product is obtained after traveling along the predetermined flow path across the sieve bend screen deck. Next, but without interruption, the now concentrated slurry moves onto and builds up on the inclined planar portion of the screen deck. Particularly at this stage it has been found that a very efficient compaction of the solids occurs and that further dewatering of the concentrated slurry now occurs. It is estimated that approximately 20% more liquid can be separated from a slurry when screened with an apparatus according to the present invention relative to conventional dewatering structures, e.g. sieve bends of comparable capacity. This intentional build up of the compacted mass is directly contrary to prior art practices as evidenced by Benzon U.S. Pat. No. 3,446,349 at col. 3, lines 5-8.
Further, the apparatus according to the present invention, due to its more efficient and economical operation, utilizes approximately only twenty-five percent of the floor space area required in a processing facility for a given feed rate when compared to a conventional pure sieve bends installation of comparable capacity. A primary beneficial aspect of the present invention is, therefore, that due to the increased efficiency and capacity a lesser number of devices according to the present invention are necessary to effectively dewater a predetermined flow of dilute slurry feed. Thus, both the initial equipment costs and operating costs are significantly decreased while requiring less floor space area when compared to conventional sieve bends of comparable capacity, and a substantially improved dewatered product is achieved.
An important variation in the use of the vibratory motion is utilized in the present invention. In conventional vibrating screens, the "throw" is customarily so arranged that the particles on the deck are literally "thrown" upward and forward in the direction of the screening flow. This is achieved through the well known balancing and positioning of the eccentric vibratory driving member with respect to the center of gravity of the screen. This conventional mode of operation is not utilized in the present invention; exactly the opposite effect is arranged and desired.
That is, the major vector of the "throw" for the present invention is upwards and backwards against the desired direction of flow of the material. The result is that the flow of the increasingly compacted mass is impeded, as against conventional screening practice, and its movement from feed end to discharge end is more of a sliding motion of the concentrated slurry. The mass is carried downward during the forward movement of the screen, as induced by the eccentric motion, but then as the screen deck is driven upwards and backwards during the "throw" of the eccentric cycle, the inertia of the compacted mass causes it to remain relatively in place and consequently effectively to more or less "slide" further down the screening deck, as the latter moves backwards.
This method of inducing movement of the mass is generally the opposite of normal vibrating screen design and practice, but is essential to achieve the unexpectedly advantageous results provided by the present invention.
Thus, it is one of the primary objects of this invention to provide a more efficient and effective apparatus for the dewatering and/or wet screening of fine grain coal particles over structures and assemblies conventionally being utilized for such purposes.
It is yet another objective of the present invention to provide a novel apparatus which can more effectively dewater and/or wet screen fine grain coal particles utilizing less floor space in a processing facility for a predetermined flow rate in comparison with conventional equipment of comparable capacity.